Answer:
a = 12 [m/s²]
Explanation:
To solve this problem we must use Newton's second law which tells us that the sum of forces on a body is equal to the product of mass by acceleration.
ΣF = m*a
where:
ΣF = sum of forces acting on a body [N] (units of Newtons)
m = mass = 0.5 [kg]
a = acceleration [m/s²]
Let's take the direction of positive forces to the right and negative forces directed to the left
2 + 8 - 4 = 0.5*a
6 = 0.5*a
a = 12 [m/s²]
Im pretty sure it’s A eye
Considering that the book is moving with constant speed, the force applied by Anna must be the same that the friction force:

If we clear the previous equation:
Answer:
Power will be 0.2023 watt
And when amplitude is halved then power will be 0.0505 watt
Explanation:
We have given mass of the Piano wire m = 2.60 gram = 0.0026 kg
Length of wire l = 84 cm = 0.84 m
So mass density 
Tension in the wire T = 25 N
Frequency f = 120 Hz
So angular frequency 
And amplitude A = 1.6 mm = 0.0016 m
We have to find the generated power
Power is given by 
From the relation we can see that power 
So if amplitude is halved then power will be
times
So power will be equal to 